Treatment of waste pickle liquor



June 7, 1960 F. D. COOPER TREATMENT oF wAsTE PICKLE: LIQUOR Filed April l, 1955 Rok* NVENTOR @aw/Ma 29W ATTORNEYS United StatesV Patent O TREATMENT OF WASTE PICKLE LIQUOR Franklin D. Cooper, Morristown, NJ., asslgnor to Otto Construction Company, New York, N.Y.

This invention'relates/to the 'production of iron-free ammonium sulfate and `iron carbonate 'through theutlization of gases essentiallycomprising'carbon dioxide and ammonia for the treatment of spent or Waste pickle liquor.

In the processing of steel, or iron-'bearing titanium ores, the disposal of waste pickle liquor presents an extremely serious problem. Waste pickle liquor is derived from sulfurie acid solutions employed in the cleaning of steel and in the extraction of iroubearng titanium ores. Ordinarily, the waste pickle liquor will contain from about 2% to about 8% free sulfuric acid, together with about to about `22% of ferrous sulfate. At the present time a large gallonage of waste pickle liquor is subjected to an expensive liming treatment to neutralize/it before it can vbe disposed of in a stream, river, or lake. In the course of this lime neutralization, the slurry produced by the liming is generally heldin large lagoons for vthe slow leaching of liquids from the .precipitated solids. Prolonged contact with air and moisture in the lagoons results in the formation of mixtures of partly soluble iron sulfates, which either retard the leaching or 'result in discoloring water `downstream from the lagoon basins. In order to meet the expense of this type of operation, efforts have been directed toward the production of a cheap building material from the solids `precipitated by the liming. To date, the results of these efforts have not been quite as successful as had beenanticipated.

Other pickle liquor disposal processes have been developed which have special requirements by Way of heat, pressure, compressed air, costly lfiltering devices and the use of hot waste slag. -There are also numerous prior processes which rely upon the use of coke oven gas 'in treating pickle liquors. Such gases contain hydrogen sultide, cyanogen compounds, an'd usually, only aboutr2% carbon dioxide. In such processes various cyanide compounds of iron are produced which make filtration of the treated pickle liquor very diflcult. Furthermore, iron sul-l fides form, in preference to iron carbonates, with the result that iron compounds of little or no commercial value are produced by such processes.

In general, processes suggested in the prior for the production of ammonium sulfate from waste pickle liquor have invariably suffered from the diculty that, in actual operation, the product ammonium sulfate contained iron. The presence of iron causes discoloration and lowers the quality and salability of such ammonium sulfate. Furthermore, the iron compounds derived from these prior art processes have so little value they are substantially waste products and their handling and other costs are not justified. Y l

Accordingly, an object of the 'present invention is to overcome these disadvantages of the prior 'art by providing a high quality, substantially pure, iron-free ammonium sulfate through the` treatment of waste pickle liquor with a gaseous mixture containing ammonia and a gas essentially comprising carbon dioxide.

Another object of this invention is to provide aprocess for the utilization ofwaste gases essentially comprising carbon dioxide, and waste pickleliquor, wherein, through the use of ammonia, with such waste gases, valuable iron products anda high 'quality iron-free ammonium sulfate may be produced.

Another object of this invention' is to provide a simple, inexpensive method for disposing of waste pickle'liquor which does not require abnormal amounts of heat and pressure, expensive equipment and control devices, or costly reagents and which, in operation, does not involve complex filtration or other handling problems 'nortlie loss of any reagent employed; such as, for example, lammonia. Y

A further object of this invention is the provision of a method for the recovery of iron-free ammoniumsulfate and iron carbonate of good quality by converting waste pickle liquor into an alkaline liquor by means of novel recycling and neutralizing techniques, and `contacting the alkaline liquor with ammonia and a sulfide-free, cyanidefree gas essentially comprising carbon dioxide so that the ammonia and carbon dioxide are rapidly absorbed in the liquor to form ammonium and carbonate ions which react rapidly with the ferrous and sulfate ionsl present in the alkaline liquor.

A still further object of this invention is to provide for the treatment of an alkaline liquor containing ferrous ions, sulfate ions and free ammonia, which liquor is derived from Waste pickle liquor, with a sulfide-free, cyanide-free gas containing above 3%; and, preferably, above 5% carbon dioxide; less than 2% sulfur dioxide, and less than 4% Oxygen, to produce iron-free ammonium sulfate,rin soluble iron carbonate and a minor amount -of iron hydrate which facilitates the separation of the iron carbonate. p

It is also an object of this invention to treat an alkaline liquor derived from waste pickle liquor and containing fer rous ions, sulfate ions and free ammonia with ammonia and a gas essentially comprising carbon dioxide to 'produce insoluble iron carbonate and an alkaline solution of ammonium sulfate which solution may be sullided while alkaline to remove all traces of ferrous` ions thus permitting the recovery of iron-free ammonium sulfate.

These and other objects of the invention may be accomplished by a process in which a gaseous mixture comprising essentially carbon dioxide and ammonia is passed in counter-current contact with a modied waste pickle liquor through at least two distinct zones or chambers. In the rst of these zones or chambers, which is identified as the alkaline section, the gas stream encounters and reacts with an alkaline feed liquor comprising partially treated waste pickle liquor containing ammonium sulfate and soluble iron salts in the form of ferro-us ions, sulfate ions and free ammonia. ln thiszone, the ammonia and carbon dioxide go into solution as ammonium carbonate and ammonium bicarbonate, which react with the soluble ferrous sulfate and other soluble iron compounds to form a precipitate of iron carbonate. Traces of iron hydrate `are formed either by virtue of the presence of up to 4% oxygen in the contacting gas or by the decomposition of traces of ferric carbonate which may form. The iron carbonate is sparingly soluble, and is readily removable by a number of conventional methods.

The gas stream -is then passed into `a second zone or chamber which is a predominantly acidiczone where all of the remaining ammonia is scrubbed from the Ygas with a mixture of spent pickle liquor and recycled partially neutralized pickle liquor to form ammonium sulfate which remains in solution. The gaseous `eilluent from the acidic zone preferably contains from 3 to 5% by volume carbon dioxide, is largely inert, and may be employed for a number of purposes in this process where' inert gasesV are desired. The iron carbonate, with traces of iron hydrates, at the most less than 2% `byweightfjof iron hydrates, is desirably separa-ted from the liquid leluen't from the rst gas contact zone. After removal of these insoluble materials, the liquor may be subjected'to further refining treatments, in order to remove any remaining f 3 traces of iron. A portion of the liquid effluent from the first or alkaline chamber is recycledA with a feed stock liquor to form an alkaline feed liquor for :the initial contact area, where the recycled eiuent contributes to the volume of the liquid, adjusts the pH of the alkaline feed liquor to between 5.5 and 9.5, and enhances the liquidgas contact in that portion of the process. The feedstock liquor is obtained, as will presently be described in greater detail, by the neutralization of the free acid equivalent in spent pickle liquor with ammonia.

Desirably, there is a complete separation between the two zones of liquid-gas contact. The liquid effluent from Vthe ammonia scrubbing acidic zone is removed firom that zone; and, preferably, a portion thereof is recycled to the ammonia scrubbing acidic zone, while the b alance is treated with ammonia, mixed with a recycled liquor containing ammonium sulfate and iron compounds, and injected directly into the initial contact zone for the gaseous mixture.

While it is usually desirable to recycle a portion of the eiuent from the primary or alkaline gas treatment zone back to that zone, a substantial portion of this liquid efiiuent may besubjectedto separation bydecantation or other conventional separation. The slurry obtained from the decantation may be subjected -to further separation operations, such as filtration, whereby solid iron cai'- b'onateV of excellent quality may be separated from the liquor. The'liquid effluent from the decantation is subjected to further rening to remove the last traces of soluble iron from the solution which is now a relatively high quality ammonium sulfate solution. The last vestiges of soluble iron are removed by treating the liquor while it is alkaline with hydrogen sulfide, whereby iron vsulfide 1s precipitated. This precipitate may be substantially removed by decantation and a polishing filter, after which the solution of ammonium sulfate may be adjusted to the desired pH by the addition of sulfuric acid. The solution is then subjected to evaporation to produce a concentrated solution of ammonium sulfate from which the ammonium sulfate-may be crystallized, dried, and stored. l Y V vIt has been found desirable in the operation described above to recycle a portion of the liquor at various points during the process. Thus, a substantial portion of the liquor from the ammonia scrubbing or acidic section of the counter-current gas-liquid contact zone may be recycled to the input of that zone. Similarly, a portion of the liquor from the carbonation or primary zone may be recycled Ito the liquid input to that zone. After the liquid efiiuent from the primary, alkaline, or carbonation zone has been decanted and filtered, a portion of the filtrate may be accumulated in a tank for recycling either to the ammonia scrubbing or acidic contact zone, or -to the primary or alkaline zone, or to both.

The iron carbonates recovered in this process may be calcined under varying conditions -to produce pigments or abrasives or may be used directly in the wet`purification of gas from hydrogen sulfide and the like. After air oxidation of the iron carbonates, the resulting material may be used for the dry purification of gas from hydrogen sulfide and the like, after mixing with shavings or other suitable diluents or binders; these mixtures may also be employed in the fiuidized purification of gas from hydrogen sulfide and the like. Thus, the iron carbonate may be effectively used in spray scrubbers without grids such as are disclosed in `the patents of `the Otto Construction Corporation for the removal of hydrogen sull fide from 'gaseous mixtures. Alternatively, the` iron carbonatesmay be sintered, with the simultaneous recovery of carbon dioxide andthe production of an' iron product which maybe employed a's a substitute' for iron ore vin the production of iron'.

The iron sulfide produced during the final purification of-the ammonium sulfate liquor with hydrogen sulfide may-,be employed as a part of the cyclic processdiscussed herein for the further production of hydrogen sulassumesY tide. Alternatively, the iron sulde may be sintered on a sin-tering belt for the recovery of the iron therein, while simultaneously producing sulfur dioxide. The iron sulfide may also be employed in the treatment of radioactive liquid wastes. 1l

The carbon dioxide-bearing gas employed in thisprocess may be obtained from a limestone kiln, a bicarbonate calciner, or from a blast furnace. The gas may also be obtained from the burning of any conventional type liquid, gaseous or solid carbonaceous fuel. The gas must contain at least 3% by volume carbon dioxide, from about 5% to about 50% by volume being quite satisfactory, and with from 8% to 25% by volume being preferred. The .gas employed in this invention must, however, be substantially cyanide-free and sulfide-free to be operative, hence unpurified coke oven gas. cannot be ernployed. Y It is possible to produce a satisfactory carbon dioxide-bearing ga's'thi'oughV the use of a special burner which burnsY short to produce one or two percent carbon monoxide, which may be converted catalytically to carbon dioxide with commercially available catalysts .for this purpose, Vsuch as iron ore and iron oxide, giving a substantially oxygen-free atmosphere. It is permissible to employ a gas in this process containing up to 4% by volume oxygen, but it is preferred to maintain the oxygen content thereofat Yless than 1.0% by volume, since the formation of small amounts of iron hydrate, in excess of 2% by Weight hinders the separation of the iron carbonate. The gas employed should notcontain inexcess of 2.0% by volume sulfur dioxide since the latter, in larger' amounts, produces ammonium suliites; and `bisulfites in amounts sufiicient 'to cause a build-up of metal sulfltes and may be reduced to form colloidal sulfur thatdeposits in filters, thus rendering them inoperative. The sulfur dioxide content of the gas may be controlled by using a fuel containing less thanabout 3.5% sulfur. It is also possible to employ blast furnace gases, or the products of combustion from the burning of blast furnace gases. Thus, rmany products of combustion may'be used, including those products of combustion from burning gaseous, liquid or solid carbonaceous fuels. This invention 'also may be carried out with artificially prepared mixtures of carbon dioxide, ammonia, and inert gases. i

A more comprehensive'understanding of this invention may be obtained by reference to the accompanying drawiileferring to the drawing, spent pickle liquor for use as feed in this process is accumulated in a suitable feed tank 46. Pickle liquor. of the. type suitable for use `in the present process ordinarily contains from about 2%, to about 15% by weight of free sulfuric acid, and from about 6% to about 25% by weight of ferrous sulfate. However, these concentrations are not critical and suitable pickle liquor for use in the present process may contain more or less of either or both of these components. Thus, for example, the spent pickle liquor feed may be derived from an ammoniumA bisulfate pickling process. Ordinarily, such a spent liquor contains no free sulfuric acid as such, but does contain a free sulfuric acid equivalent, in the `form of unreacted ammonium bisulfate in low concentration. Such a spent pickle liquor is quite suitable for use in the present process, which isparticularly designed for the; treatment'of acidic solutions containing ferrous ions and sulfate ions. .p

From'the feed tank 46 thepickle liquor is delivered by a pump 54 through-a-conduit 29 and a mixing nozzle 56 into a counte'rcurrent gas-liquid contact column 32I In the mixingV nozzle 56, the spent pickle liquor `is interfmixedwith recycled, partially'neutralized spent pickle liquor containing ammoniumsulfate, 'ferrous ions,` and sulfate ions, -to form anv acidic feed liquor for the gasliquid contact column 32. I Y,

From the mixing riozz1`et he liquid input (C) Vis sprayed into the top of the ycoun'tercurrent contact ,column 32 which is hereinafter designated as the reactor-carbonator. The reactor-carbonatr ,3.2 may be anysuitable type of 15 'contact titower :for Aeiecting intimate contact between l the feed liquor'and the gas stream. For example, `the reactor- `-'farlronator 32.may be a substantially empty metal shell, equipped at the top with a spray distributor 201 for the feed liquor, and an' entrainment arrester bed 202. eramimmetallic, `or any `other type of spiral or other packing may be employed `for improving thegas-liquid Contactin the column and for construction of the entrainmentarrestor bed 202. Theupper portion of the reactor-carbonator 32 is an acidic section whose Aprimary function is to scrub any remainingtraces of ammonia from the gas which is about 'toleave'the column. The lower portion of the reactorcarbonator in` an 'alkaline section in which the formation oflammonium'sulfate and iron carbonate is accomplished. Idemarcation between the upper or `acidic section and the lower or alkaline section of the reactor-carbon'ator is provided by a hooded cap tray 2M. On this '.tray, theacidic elhuent liquor (E) from theacid section of the reactor-carbonator is removed and is transfel-red `into a pump tank 150.

"Since it is highly desirable to avoid contact between 'the `liquor and air, in order to avoid oxidation of the iron compounds, `the liquor is preferably `blanketed in Ythe pump tank with `an inert` gas, and, as well, wherever possible throughout the system. Inert gas for the entire system may be provided by pumping any portion of the discharged gas from the reactor-carbonator 32 into a Igasometer 31, where it may be stored `under pressure.

` 'The inert gas for the pump tank is desirably obtained Ifroma source occurring later in the system. After the iron carbonate slurry has been obtained 'by decantatio'n, this slurry is filtered to obtain an iron carbonate lilter cake 'andan ammonium sulfate liquor. The lltration is preferably accomplished under a blanket of inert gas from the gasometer 31, and is expedited by the use of a vacuum, in order to achieve a high ltration rate. The 'off-gases lfrom the vacuum pump contain inert gas from the `gas blanket, and ammonia which has escaped from the liquor because of the reduced pressure. These offgascs are desirably employed to provide the inert gas blanket in the pump -tank 150, Where the ammonia is absorbed as the gas bubbles up from the sparger 251 through the acidic liquor.

A portion of the contacted liquor (E) from the pump tank 150 is delivered through the recirculation pump 151 back to the'mixing nozzle 56, thus providing a recirculation which `increases the volume of the acidic solution in lthe acidic section of the reactor-carbonator. This increased volume enhances the gas-liquid contact. The pump 'tank 150 is also provided with an voverlow Weir 252 V'which discharges by gravity through a seal pipe 253 `into an interneutralization tank 35. Uunder `normal perating conditions, the liquid overflow leaving the pump tank l15) through the seal pipe 253 is always equal to the volume of liquor which passes through the mixing nozzle 56, exclusive of the recycled amount from the pump tank 150.

The interneutralizatio'n tank 35 receives the contacted acidic liquid eiiiuent (E) from the pump tank 150, and also allarge 'amount of recycle liquor (R) containing ammonium rsulfate and soluble iron from a recycle tank 50, which is delivered by means of a pump `55 through a suitable conduit 52. These two liquids are mixed in the interneutralization tank 35 by a stream of inert gas which is'admitted through the sparger 36 at the bottom of the tank. Controlled amounts of ammonia may be mixed with the inert gas by means of an automatic valve and the mixture of ammonia and inert gas is then introduced into the -tank 35 to reduce the acidity of the liquor. These are several purposes for the addition of ammonia at this point. An important purpose is to control the pH of the liquor so that it is `within a range where the formation of insoluble iron compounds is minimized, `thus Vfacilitating spraying into the alkaline (lower.) section of the f6 reactor-carbonatorzg.At a `pH of 5.5 .or lower, no ferrous hydrate `will form, but the maintenance of a-pH lower than about 5.5 precludes the addition of anysubstantial amount of ammonia in the Vinterneutralizaton tank 35. Another important purpose of ammonia-addition in the interneutralizing tank isto provide a liquor of the proper pH in the reactor-carbonator for absorption of carbon dioxide. At atmospheric pressure, carbon dioxide is not readily absorbed in acidic solutions. When the pH ofthe liquor is adjusted vto `about 5.5 to 6.0, in the interneutralization tank, the combined action of admixture with alkaline recycle liquor `and injection into an atmosphere-rich in ammonia and. carbon dioxide,

` promptly producesammonium carbonate and bicarbonate in solution. This leads to the desired formation of iron carbonate by 'reaction with the soluble iron in the solution. in the range of 5.5 to 6.0, the formation of solids in the interneutralization tank is minimal, and Spraying the liquor into the alkaline sectionofthe reactor-carbonator is a simple operation.

For best operation, it is desirable to distribute the ammonia addition to the.p'r`ocess between the gases entering Vthe alkaline section of the reactor-carbonator and the liquor -in the interneutralization tank. Regulation of the pH of the liquor in the interneutralization tank to about 5.9 usually results in a highly satisfactory distribution. Atthis pH, Vthe formation ofhydrates is inconsequential, but a considerable amount of ammonia can be absorbed. The automatic valve 5 through which ammonia is introduced into the inert gas is controlled by a .pH responsive control 6 having electrodesin the discharge line 40 `from the tank. The inert gas in the mxture thereof with ammonia excludes air from the intervneutralization tank, improves gas-liquid contact and provides Vthorough `mixing in the tank 35.

From the interneutralization tank 35 the Iammoniated liquor, hereinafter designated as the feed stock liquor (S), is delivered by a pump 39into the'alkaline section of the reactor-carbonator through conduits 40 leading to a group of spray distributors 33 which are arranged in parallel. Before `entering the reactor-carbonator 32, the feed stock liquor combines with recycled liquid effluent (B) from the alkaline section of the reactor-carbonator, to form an alkaline feed liquor (A). The alkaline feed liquor (A) desirably has a pH in the range of about 7 to about 8, although it may be in the range of 5.5 to 9.5 While starting up, or during exceptional operating periods. Desirably, the conduits 40 are provided with heat exchangers 41 for controlling the temperature of the alkaline feed liquor (A) as it enters the alkaline section of the reactoricarbonator.

In the alkaline section of the reactor-carbonator the alkaline feed liquor (A) comes into intimate contact with gas containing both carbon dioxide and ammonia. It yis probable that, `in the alkaline section, the ammonia and carbon kdioxide react in aqueous solution to form a solution of ammonium carbonate and ammonium bicarbonate, which, in the presence of ferrous sulfate and ad- `ditional free ammonia, form ferrous carbonate, traces of ferric Acarbonate and ammonium sulfate. These iron compounds `are only sparingly soluble and precipitate out of .the solution. The contacted liquor (B) leaving the reactor-earbonator 32 through the bottom header 42 is, therefore, a solution of ammonium sulfate containing a suspension of insoluble ferrous carbonate, `suspended traces of ferrie carbonate and otherinsolubles, and slight amounts `of soluble iron compounds. A portion of this may be recycled to the reactor-carbonator by passing it through the pump 43 and the recycle conduit 44 to the heat exchangers 41. The recycled contacted liquor (B) increases the volume of liquid in the alkaline section and thus enhances the gas-liquid contact inthe reactorcarbonator.` The recycle should be between 50% and by volux'nevof the total amount of contacted liquor (B) leaving' the reactor-carbonato; The balance of the substantially free of sulfur dioxide.

alkaline feed liquor raises the pI-I of this liquor into the range 7 to 8, under good operating conditions. The recycling of precipitated carbonate, in suspension, also promotes the formation of relatively vlarge particles of precipitate, whichrfacilitates later separation of the precipitate.

In order to produce the insoluble iron carbonates in the liquid effluent (B) from the alkaline section, the gas employed must be rich in carbon dioxide and the proper pH must be'maintained in the solution by the use of ammonia. Better operating conditions'are obtained if fthe ammonia additions are properly proportioned between the interneutralization tank 35 and the reactor-carbonator 32. Thus, the ammonia addition in the interneutralizing tank 35 is desirably controlled automatically so that the pH in the feed stock liquor (Si) is about 5.9. Ammonia may be added to the reactor-carbonator in a gaseous admixture with the carbon dioxide and inert carrier or inert diluent gases, if any, through the inlet gas header 34; or, alternatively, the ammonia may be iniected through a series of injection pipes 37, arranged beneath the alkaline feed liquor sprays 33. In either case, the addition of the ammonia is desirably controlled by` a valve controlled automatically by a pH responsive control 6. 'The carbon dioxide content of the inlet gas (total amount to the reactor-carbonator 32) should desirably be about 3% to about Y50% by volume. A sucient concentration of carbon dioxide Yshould be employed so`that there is a detectable amount of carbon dioxide present in the discharge gasesfrom the reactorcarbonator. While carbon dioxide contents as low as 3% by volume of the input gas may be employed, 8% or more byV volume of carbon dioxide is desirable. Gases of this carbon dioxide content are readily available from a plurality of sources. For example, any of the common industrial fuels will normally yield a stack gas containing at least 8% carbon dioxide by volume. Where the input gas contains small amounts of carbon monoxide, this may be catalytically converted into carbon dioxide by a plurality of catalytic reactions in common use for that purpose. This step has the added advantage of providing the reactor-carbonator with an oxygen-free atmosphere. -In any case, it is desirable to employ a gas having as little oxygen as practicable. The presence of oxygen would. lead to the undesirable oxidation of iron compounds to give large amounts of insoluble iron hydrates. Since these are not as readily separable from the lliquid as are the carbonates, their formation should be minimized.

It is essential for successful operation that the gas be The presence of quantities of sulfur dioxide in the input gas is harmful and undesirable for several reasons. Sulfur dioxide, in amounts over about 2% by volume, produces ammonium sulfites, bisuliites, or thiosulfatcs in undesirably large quantities. This results in contamination of the iron carbonate, which renders it less desirable commercially and unsuited for many uses. These sulfur compounds are often reducedduring the process, with the resultant undesirable formation of colloidal sulfur which contaminates the iron carbonate, and which deposits in the screens and filters and renders them inoperative. However, a sulfur dioxide .content in the input gas of about 2% by volume, such as would be'obtained vby burning strip coal, 'is tolerable. The absence of cyanides in the input` gas is also essential, since the complex cyanides which form during the vprocess render filtration a practical impossibility.

The balance ofthe liquid Yefiiuent from the alkaline section of the reactor-carbonator is deliveredvby the pump 43 previously mentioned through a pipeline 45 into a large decanter tank 57. The pipeline 45 between the delivery pump 43 and the decanter'tank 57 is desirably provided with a heat exchanger 255, for standardizing settling temperatures. The efuent transferred into the decanter is a slurry, and it is admitted to the decanter 57 through a conduit 45- which discharges near the midpoint of the total working depth of the decanter against a horizontal defiector plate 256. Insoluble solids inthe yinput slurry settle toward the conical bottom 127 of the decanter, and the solids are withdrawn through a discharge conduit 60. The solids settling at the decanter bottom 127 are in the form of a thick slurry which is transferred by a pump 66 into a filter 70. Alternatively, a thickener (not shown) may be interposed between the filter 70 and the pump 66 to decrease the load on the filter. The filtrate and any washings produced in the operation of the filter are returned to the recycle tank 50, for reprocessing. l

The `iron carbonate filter cake from the filter 70 is re'- moved for further processing such as drying and sintering. Alternatively, the filter cake may be processed in a slurry feed preparation tank, prior to use in the purification of various gases. The filter cloth may be freed periodically of slimy insoluble iron precipitates by washing with acidic spent pickle liquor. This prevents cloth blinding. These washings are desirably returned directly to the pickle liquor storage tank 46 for processing through the reactor-carbonator (flow not indicated in the diagram), or, alternatively, the washings may be added to the filtrate stream going to the recycle tank 50. The cake itself may be washed with water, and the washings are desirably transferred into the recycle tank 50 for processing in the interneutralization tank 35.

The liquid efl'iuent from the decanter 57 is taken off over a Weir 257 at the top of the decanter, `from which it flows into an accumulator or surge tank 58. The rate of flow of clarified liquor (D) from the decanter is equivalent to the input of spent pickle liquor feed processed, plus washings from the filter cake, plus recycled clarified liquor which had been returned to .the reactorcarbonator for reprocessing from the accumulator tank 58, by Way of the recycle tank 50, and through either the acidic section of the :reactor-carbonator or the in- 'terneutralization tank 35. Regulation of the flow into the sulfiding tanks is accomplished by la piping arrangement in which a pump 59 may transfer up to a certain rate of fiow to the sulliding tanks with any excess being transferred into the recycle tank 50. The accumulator tank 58 serves to smooth out flow in the system, (since any sudden surges of liquid into the accumulator tank are routed into the recycle tank 50.

The ammonium sulfate solution (D) which has been transferred into the sulfiding tank 79 contains small amounts of soluble iron. In order to remove these amounts of soluble iron, gaseous hydrogen sulfide is introduced into the tank through a sparger 82. Iron sulfide is formed and is allowed to settle to the bottom of the treatment tank for removal. In order that the operation may be continuous, a plurality of treatment tanks may be employed in parallel relationship. While the source of the hydrogen sulfide may be an independant generator, or a gas bottle, `it is advantageous to collect the settled iron sulfide in a vgenerator 86 for regeneration. Hydrogen sulfide may be produced in this generator by treating the iron sulfide with fresh pickle liquor pumped through a line 8S from the spent pickle liquor feed tank 46 by thefeed pump 54. Alternatively, other acidic materials may be employed for the regeneration.

In order to avoid the formation of iron hydrates in the sulfiding tank 79, a stream of inert gas is desirably introduced into the tank continuously through a line 87 into the sulfide generator 86 and the spargerZ. The agitation derived from the inert gas has a beneficial veffect in the occulation of the iron sulde, .and assists in forming precipitated particles of a desirable size. The

continuous stream of inert gas also dilutes the hydrogen' sulde, thereby aording a more intimate contact between the solution and the sulde gas. The formation of hydrates at this point is particularly 4to be avoided, since hydrate formation Would make the settling and ltration of the solution very diiiicult and slow, would cause the waste of hydrogen sulfide gas, and would render unavailable the regeneration system just described. The inert gases may be vented continuouslythrough vents 89 at the top of each suliiding tank. When any of the tanks are empty, between batches, the flow .of inert gas should be continued in order to avoid the entrance of air. When a tank is emptied, the ilow of inert gas should be sufficient to prevent the entrance of air into the tank.

After settling has taken place in the treatment tank 79, the supernatant liquor is pumped through a polishing filter 99 which removes all traces of solid material from the liquor. The liquid which has passed through the polishing lter 99 is clear, free of insoluble materials, and is free of grease. This liquor is collected in iinal neutralization tanks 101, in which .the liquor is treated with a controlled amount of iron-free sulfuric acid from a storage tank 106 in order to convert any :free ammonia to ammonium sulfate. The treatment with `the iron-free sulfuric acid may be continuous and is regulated so that a pH in the range of 6.0 to 9.0 is obtained. The liquor is then subjected to evaporation in a conventional multiple-eiect evaporator 109, which supplies vconcentrated ammonium sulfate liquor to the crystallizer 1.11. The Wet `crystals and magma from the I0 crystallizer 111 arefcentrifuged, and the crystals are dried and stored. These product crystals are substantially free of any contaminating iron, grease, or other `undesirable substances.

The precedingdescription of the process, as Villustrated in the tiow sheet, will make clear the underlying principles of the present invention. it will be understood that many modifications may be made in the Iprocess which are within the scope of the invention. A few such modications will appear in the processes described in detail in Table i. Thus, for example, in the relatively small scale operations described in Table I, where the volume of spent pickle liquor treated per hour was less than 150 gallons per hour in each case, the volume "of pickleliquor treated by the process was increased by the amount of water employed to Wash the iilter cake. The washings from the'tilter cake, including about 95% of the ammonium sulfate `retained on the cake as well as the wash water, were collected in the recycle tank and, from the recycle tank, were transferred into the interneutralization tank 35, thence into the alkaline section of the re- .actor-carbonator. As can be seen from Table I, the

amount of Awater employed to wash the iilter cake may be a relatively `small amount, or it may be sufficient to add substantially to the total amount of pickle liquor treated in the process.

The following data were obtained from a relatively small scale operation and Willserve to illustrate the operation of the invention.

TABLE I REAOTOR-CARBONATOR GAS BALANCE Example 1 2 3 `4 5 6 l. B.c.i.h. G01-Bearing Gases Entering Reactor-Cannoni ator at F. 30" Hg sat. with water (NH3-Free Basis). 4, 000 4, 000 4, 000 3, 000 3, 000 3, 000 2. Vol. Percent C01 in Gases Entering Reactor-Carbonator (NH3-Free Basis) 10.00 .10. 00 10.00 21.00 21.00 21.00 3. Lbs. NH3 per hour in Gases Entering Alkaline Section of Reactor-Oarbonator 18. 18. 82 18. 80 38. 73 38. 83 38. 54 4. Gases Entering Reactor-Carbonator:

Total c f h 4, 420 4, I 4, 420 3, 866 3, 866 3, 866 NHT-c1211-, 420 420 420 866 866 866 CO2- o t' h 400 400 400 630 630 630 5; Gases Leaving Alkaline Section of Reaetor-Carbonator:

Total c Lh 3, 816 3, 816 3, 816 2, 598 2, 598 2, 598 NH3-o f h 16 16 16 18 18 18 CO2-c i h 200 200 200 210 210 210 6; Lbs. N'Ha per hour in Gases Leaving Alkaline Section oi Reactor-Carbonator 0. 72 0.72 0.72 0.81 0. 81 0. 81 7. Total c.f.h. Gases Leaving Acid Section of Reactor- Carbonatnr 3, 800 3, 800 3, 800 2, 580 2, 580 2, 580 8. Vol. Percent COiin Gases Leaving Reactor-Carbonator (NH3-Free Basis) 5. 26 5. 26 5.26 8.14: 8.14 8. 14

REACTOR-CARBONATOR LIQUID BALANCE `9. G.p.h. Spent Pickle Liquor Feed To Acidic Section of Reactor-Carbonator 60. 0 33.3 43.8 138. 6 70. 0 91. 9 10. Analysis of Pickle Liquor:

(a) (G.p.l.7 FeSO4) 146 288 220 146 288 220 (b) (G.p.l. H1804) 51 21 108 51 21 108 11. G.p.h. at rfromSDlsplacement Washings of FeCOg Solids 22. 1 22. l 22. 1 46. 4 46. i 46. 4 12.- G.p.h. Filtrate Washings Recycled to the Interneuv tralization Tank 122.6 133. 2 175. 2 257. 4 280. 0 367. 6 13. G.p.h. Feed to Alkaline Section of Reactor-Carbonator i (Exclusive of recycled eilnent from the alkaline section) 188. 6 166. 5 219. 0 396. 0 350. 0 459. 5` 14. Feed to Alkaline Section of Reactor-Carbonator:

(a) Vol. Percent Neutralized Pickle Liquor 35 20 20 35 20 20 Vol. Percent Recycled Separated Liquor 65 80 80 65 80 80 (c) G.p.l. FeS04 Equ.iv 57. 6 65. 6 52.0 57.6 65. 6 52.0 16. Lbs. HiSOi Entering System Per Ho 27.83 5.83 39. 42 58. 90 12. 25 82.71 16; Lbs. H2304 Per Hour Reacting with H3 Leaving Alkaline Section of Reactor-Carbonator. 2.07 2.07 2.07 2. 33 2. 33 2. 33 17. Lbs. H2804 Per Hour Reacted by NH3 in Recycled Liquor and by direct NEM, addition in interneutrali ization tank 25. 76 3. 76 37. 35 56. 57 9.92 80.'.38 18. G.p.h. Reactor-Carbonator Etluent (B) to Decanter (Excludes amount recycled back into the alkaline Section) 196. 0 178. 1 228. 5 411. 4 373. 8 479. 4 i9. G.p.h. Potential Filtrate iu Reactor-Carbonator Emuent Liquor\(B) 171. 0 149. 5 `203. i 358. 7 318. 5 424. 7 20. Composition of Euent Liquor (B) from the Reaetor- Carbonator:

(a) G.p.l. FeSO4 Equiv 10.0 10.0 10.0 10.0 10.0 10.0 (b) G.p.l. Free NH3 2.0 2.0 2.0V 2.0 2.0 2; 0 21. Vol. Percent Potential Filtrate in R-O Eflluent Liquor (B) 87.2 1 89.0 .2 86.1 89.0

` ee footnote atend of table.

TABLE L-Continued FILTER CAKE Example 1 l 2 3 4 5 6 22. Lbs. Per Hour Wet Washed F500. Produced 125. 125. s 126. o 255,7 Y 254.1 254. s 23. Wet Washed FeGOa Cake Composition:

(a) Percent A.S.T.M. H 48.5 48.5 48.5 48.5 .48.5 48.5

(b) Percent FeCOa Equiv 48. 5 48. 5 4.8. 5 48. 5 48.5 48. 5

(a) Percent `(Numero, 3.0 3.0 3.0 5.o 3. 0 3.o

SULFIDING OPERATION 24. G.p.h. Filtrate to Sulder 70. 5 38.4 25. Grams FeSO4 Equivalent Sulded Per Hour 2, 668 1, 435 26. C.f.h. E1s Reqd for Sulfiding FeSOl only 14. 7 7.9 27. G.p.h, Original Pickle Liquor Reqd to Produce H 11 40 for FeSO4 Equiv. Removal 8. 96

53. a 147.7 s4. 9 103. 5 2, 02o 5, 59s 3,216 3,922 11. 2 so. Q 17. e 21.1

a 2o 18.70 1o e 19 AMMONIUM SULFATE RECOVERY Y NH3 Recovery as Salabla (NHmSOl 96. 6

Example 1 Referring to Example 1, that operation of the process was one in which the ammonia addition was distributed between the interneutralization tank and the stack gas which was employed as gaseous feedfor the reactorcarbonator. The volume of carbon dioxide gas employed per hour, 4,000 standard cubic feet, is on an ammoniafree basis. On this same basis, the volume percentage of carbon dioxide in the stack gas was 10.00%. This was within the optimum-operating range of 8 to 25 volume percent of carbon dioxide. This percentage represented the average of a number of readings taken continuously during the operation. The same is true of other figures in Table I. Starting up and shutting down periods were g excluded in computing the average in order to obtain typical operating data. Y

In Example 1, all of the ammonia added to the re- Y actor-carbonator was gaseous ammonia and was Yadded to the carbon dioxide-bearing gas immediately preceding the injection of that gas into the reactor-carbonato'r. 18.80 pounds of ammonia per hour was the rate of feed of ammonia into the reactor-carbonator, and the greater percentage of this ammonia was absorbed or reacted in the alkaline section of the Vreactor-carbonator. Only 0.72 pound of ammonia per hour left the alkaline sectionY of the reactor-carbonator and entered the'acidic section.

'Following the flow,` of liquid through the system, theY net throughput of the acidic section of the reactor-carbon-V ator was equal to the feed rate of spent pickle liquor, 66.0 im

small amount of pickle liquor from the generation of'" hydrogen sulfide.

In each of these examples, all of the recycled liquid from the recycle tank 50 was transferred into the interneutralization tank 35, in preference to recycling with the spent pickle liquor feed going into the acid section of the reactor-carbonator. The rate of Viiow of the recycled' liquor (R) into the interneutralization tank was 122.1 -gallons per hour (line 12).- VThis inw?-Y creased the net `throughput of the alkaline section of the 35 tion was comparatively large.

':- canting. i' Together .withthe spent pickle liquor employed reactor-carbonator to 188.6 gallons per hour.v .,Sincer 65% by volume of the total throughput ofthe alkaline section comprised recycled effluent from the alkaline section (line 14b) whereas the ne throughput of 122.1 gallons per hour constituted only by volume of the total'(1ine 14a), Vthe totl'throughput for the alkaline sec- Because of the absorption of ammonia and carbon" dioxide in the alkaline section,

y with the subsequent formation of ammonium sulfate and ironcarbonate, the amount of etlluent from the alkaline Y section is larger than the amount of input liquor. In Ex- 40, ample l, the input to lthe alkaline section was 188.6

gallons per hour, whereas the output was 196.0 gallons "perhour (line 18), an increase of 7.4 gallOIlS Per hour directly attributable to reactions taking place in the alkaline section.

5 The 196.0 gallons per hour of alkaline section efiluent A'v tion is not obtained, and4 some of the ammonium sulfate solution is entrained in the liron carbonate filter cake. VThe supernatant-.liquor-from the decanter was divided f into vtwo portions, v70.5 gallons per hour going to the suldingV tanks (line 24), and the balance being transferred into the recycle tank 50, and from there to the in- `'tcrneutralization tank 35. The heavy iron carbonate .v Yslurry obtained inthe bottom of the decanter was filtered to obtainV a clear. filtrate and a filter cake. The clear iiltratel was transferred, in its entirety, into the recycle tank 50, for reprocessing in the interneutralization tank.

" Thfilte'r cake was washed with three displacement washes of` water, and the washings thus obtained were also ...transferred into the lrecycle tank for eventual reprocessing in the interneutralization tank. The filter cake was produced' at the rate Vof 126.5 pounds of wet cake per J hour' (line V22). Approximately 95% of the ammonium sulfate entrained in vthe cake was removed by the water -gwashing andl was recovered from the washings. The wet cake contained about 61.4 pounds of water per hour,

'{0'65485-25 by weight'of the wet cake (une 23a). The

*recycle tank tlth'erefore received liquor from the filtering operation,.t he washing operation, and from the de- -from timeY to time in the hydrogen sulfide generator, this 5 recycle 4liquor Vwas returned to the interneutralization 1.3 tank for the recovery of ammonium sulfate contained therein, and for the purpose of increasing the total throughput of the alkaline section, for the many purposes already mentioned.

The sulding operation `removes all remaining traces of soluble iron in the solution of ammonium sulfate with substantially no change in the volume of the solution. The output from the sulliding tanks is therefore approximately equal to the .input to the sulding tanks. An amount of iron equivalent to 2,668 grams of ferrous sulfate was removed each hour in the sulfiding tanks.

The spent pickle liquor employed as feed in each of the examples reported in Table I was obtained from a sulfuric acid pickling bath. On the average, in Example l, the spent pickle liquor feed in the feed storage tank 46 contained 51 grains per liter of sulfuric acid, and 146 `grams per liter of ferrous sulphate or its equivalent (line b). The spent pickle liquor was introduced into the process through the acidic section of the reactor-carbonator, then passed into the interneutralization tank 35. The total input of sulfuric acid to the process per hour in Example l was 27.83 pounds (line 15,). In the acid section of the reactor-carbonator, 2.07 pounds of sulfuric acid per hour were consumed in scrubbing ammonia from the discharge gases (line 16). In the interneutralization .tank, the remaining 25.76 pounds of sulfuric acid (line 17) were essentially neutralized by the addition of suicient ammonia in that tank to adjust the pH of the eluent feed stock liquor to approximately 5.9. Complete neutralization was achieved in the reactor-carbonator, where sufficient ammonia was injected to raise the pH of the effluent liquor (B) to a pH of 9 to 9.5. The euent liquor from the alkaline .section contained only about 10.0 grams per liter of ferrous sulfate equivalent (line 20a), as contrasted with the 146 grams `per liter in the spent pickle liquor feed to the process (line 10). The eluent from the alkaline section also contained approximately 2.0 grams per liter of free ammonia. The excess of free ammonia in the liquor maintained the pH in the desirable range of 9 to 9.5 throughout the rest of the system, that is, during the decanting of the ferrous carbonate, and during the sulfiding operation.

The iron sulde produced in the sulfiding tanks was used .to produce `hydrogen sulfide for treatment of succeeding batches. If this could have been done on a continuous basis, it would have required about 8.96 gallons of spent pickle liquor per hour to decompose the iron sulphide. Nearly quantitative recovery of hydrogen sulde was obtained by the decomposition.

Since there is substantially no change of volume in the sulfiding tanks, the same output was obtained from the tanks as was put into the tanks; namely, 7.0.5 gallons per hour. This output was substantially iron-free, and was treated with iron-free sulfuric acid to bring the pH of Athe solution within the range of about 6 to 9. The solution was then concentrated by evaporation, and crystallized to obtain iron-free ammonium sulphate. The recovery of ammonia as salable `ammonium sulfate was equivalent to 96.6% by weight of the ammonia input.

This operation `produced, 126.5 pounds of ferrous car bonate filter cake per hour. The cake contained less than 3% by Weight of ammonium sulfate, and more than 97% Wet ferrous carbonate cake. This cake was approximately equally divided by weight between entrained Water and the ferrous carbonate solids.

Example 2 Since the spent pickle liquor contained only approximately 21 grams per liter `of free sulfuric acid, the total amount `of sulfuric acid entering the system per hour was only 5.83 pounds. -In order to compensate in part for the smaller total throughput of spent pickle liquor, the recycle ratio in the acid section of the reactor-carbonator was increased. In the alkaline section, by volume of the eflluent from the alkaline section was recycled thereto as a part of the alkaline feed liquor.

Although the total input to the reactor-carbonator was considerably less than in Example l, because of the higher concentration of ferrous sulfate equivalent in the feed liquor, the amount of ferrous carbonate cake produced per rhour was approximately the same, being 125.8 pounds per hour. It Was of substantially the same quality as that produced in Example 1. The overall ammonia recovery as salable ammonium sulfate was 95.3, which is comparably high with the recovery obtained in Example l.

Example 3 In Example 3, the input to the acid section of the reactor-carbonator of spent pickle liquor was 43.8 gallons per hour. This hatch of spent pickle liquor contained 220 grams per liter of ferrous sulfate equivalent, and 108 grams per liter of free sulfuric acid. The recycle ratio in the alkaline section was maintained at 80% by volume of the total input to the alkaline section. Because of the very high concentration of free sulfuric acid in the spent pickle liquor feed, the total amount of sulfuric acid entering the system per hour was 39.42 pounds. In this third example, the ammonia recovery was 97.1% efficient, as salable ammonium sulfate. The higher recovery is probably attributable, in part at least, to the high concentration of free sulfuric acid in the spent pickle liquor feed.

Example 4 In Example 4, the s pent pickle liquor feed had the same analysis as that used in Example l. The input to the acid section Was increased to 138.6 gallons per hour of spent pickle liquor. There was also added to the system, from the Vrecycle tank 50, 46.4 gallons per hour of wash water from the lter cake Washing, together with displaced ammonium sulfate solution. The `high input volume of spent pickle liquor produced 265.7 pounds of wet ferrous carbonate cake per hour. The ammonia recovery was 96.5% ecient as salable ammonium sulfate. Because of the high input rate, the recycle ratio in the alkaline `section of the reactor-carbonator was dropped to 65% by volume of the total input to the alkaline section.

Example 5 In Example 5, the spent pickle liquor employed had the same analysis as that used in Example 2. The input to the reactor-carbonator of spent pickle liquor was regulated to 70 gallons per hour. 46.4 gallons per hour of Wash Water used to Wash the ferrous carbonate filter cake, together with displaced liquor, were added to the system. This produced 264.1 pounds per hour of wet ferrous carbonate filter cake, and resulted in a 95.1% recovery of Vthe ammonia as salable ammonium sulfate.

Example 6 In Example 6, the spent pickle liquor feed had the same `analysis as that employed in Example 3. The ow rate was set to 91.9 `gallons of spent pickle liquor per hour to the acid section of the reactor-carbonator. This was augmented by the addition of 46.4 gallons per hour of wash water used to wash the filter cake, together with displaced ammonium sulfate solution, which were added to the ,interneutralization tank. In both Examples 5 and 6, because of the relatively low throughput, the recycle ratio in the alkaline section was set at 80% by volume of the total feed to the alkaline section. In Example 6l this produced a wet ferrous carbonate cake amounting to 264.3 pounds per hour, and resulted in an ammonia recovery of 96.9% as salable ammonium sulfate.

While the process which has been described is extremely flexible, the process depends for its success on the occurrence of certain chemical reactions, and care must be taken to employ suitable reagents and to adjust the reaction conditions throughout the process in order that the desired reactions take place. Thus, the process is designed to recover valuable components from acidic Waste liquors, and is equally eflicient for the recovery of vammonium sulfate from spent pickle liquors derived from sulfuric acid pickling solutions and from ammonium lbisulfate pickling solutions. While a typical pickle liquor from a sulfuric acid pickling process will contain free sulfuric acid together with ferrous sulfate, inhibitors, and other materials, the spent pickle liquor from a bisulfate pickling solution will contain a small amount of ammonium bisulfate, ferrous sulfate, ferrie sulfate, manganese sulfate, and copper sulfate. The amount of ammonium bisulfate present in the spent liquor measures the free acid activity of the spent pickle liquor. In extreme cases, it may be desirable to ladd a small amount of free acid to the spent pickle liquor to facilitate operations. .The process is fundamentally the same with either type of feed liquor, despite the minor differences in actual composition.

Pickling operations are notoriously erratic and seasonal, and Vthe present process is specifically designed to accommodate fluctuations in the amount and typerof available raw. material. Where only a small amount of spent pickle -liquor solution is available, land it is necessary to decrease the feed rate into the reactor-carbonator to a low amount, the total amount of liquid passing through the reactorcarbonator may be maintained at a normal level by increasing the amount of recycle. Additional Volume of liquid is provided by employing Water' for washing the filter cake of iron carbonate. The wash water and displaced solution may be added directly to the interneutralization tank for treatment with ammonia, with subsequent injection into the alkaline section of the reactor-carbonator. For example, in a typical installation of this process at a pickling plant, the typical normal feed rate of spent pickle liquor to the -acid section of the reactorcarbonator might be on the order of 500 gallons per hour. Arr additional 200 gallons of water could be employed to Wash the lter cake, and thewashings would be added to the alkaline section of the reactor-carbonator after treatment with ammonia in the interneutralization tank.

As has previously been pointed out, the gas stream fed intothe reactor-carbonator may be obtained from a plurality of sources. An oxygen-free atmosphere is conductive to the formation of ferrous carbonate of easily iltera-ble characteristics, free from the slime which is characteristic of iron hydrate, and which is lilterable only With great ditiiculty. The burning of fuel to provide a gas for lthe reactor-carbonator affords operational economies since the heat produced in the burner may be employed to operate the heat exchangers in the process where necessary, and to operate the evaporator and dryer in the final processing of the ammonium sulfate solution.

Whatever the source of gas for the reactor-carbonator, it is desirable that the cai-bon dioxide be provided in a considerable excess over'that necessary Vin the process. Theexcess of carbon dioxide tends to drive the reaction in the desired direction, and is a desirable component of the inert gas employed to blanket solutions throughout the process.

The acid section of the reactor-carbonator functions as a scrubber since it removes any traces of ammonia from the gas, thereby providing `an lextremely economical operation. The liquid efuent from the acid section ofthe reactor-carbonator is acidic in nature, and in order to remove iron from this solution, as a carbonate, it must be rendered alkaline. There are several possible ways inV which to render the solution alkaline. It can be accomplished by injecting all the necessary ammonia along with the carbon dioxide bearing gases entering the reactor-carbonator. Alternatively, all of the ammonia could be injected at a plurality of levels Within the reactorcarbonator. Each of these lalternatives imposes great demands on the reactor-carbonator, making its design and operation difficult. As another alternative, all the ammonia could be added to the liquor immediately preceding injection of the liquor into the reactor-carbonator. Similarly, any combination of the preceding alternatives could be employed. Each of the preceding alternatives represents a possi-ble method for operating the process, but eachl represents an extreme. For example, the injection of all of the ammonia into the liquor immediately preceding delivery of the liquor into the reactor-carbonator would raise the pH of the liquor, and would create a region of high ammonia concentration in the reactor-carbonator at the point or points of liquid delivery to the reactor-carbonator. ln this region, ferrous hydrate would tend to form because of the high pH be- Yfore the carbonate -ion in solution became sufficiently concentrated to cause-the formation of any substantial amount of ferrous carbonate. The result is invariably, under the operating conditions, the occlusion of ferrous hydrate Within a ferrous carbonate precipitate. Where all of the ammonia is introduced in the reactor-carbonator, there is an -admixture of a very alkaline recycle liquor with a highly acidic spent pickle liquor, immediately prior to the delivery of the feed liquor to the reactor-carbonator. Although the mixture of liquors, if thoroughly mixed'and reacted, is alkaline, under operating conditions localized acidic pockets form, .with the consequent undesirable dissolution of ferrous carbonate from the recycled liquor. Furthermore, if a feed liquor of low pH is delivered to the alkaline section of the reactorcarbonator, the design must provide sucient'time of passage in the tower for suicient ammonia absorption by the liquor to raise the pH to an effective level for the yformation of ammonium carbonate and ferrous carbonate.

The preferred method of ammonia addition is one in which the addition is distributed between injection in the interneutralization tank and gaseous injection into the reactor-carbonator, preferably along with the carbon dioxide bearing gases. -In the interneutralization tank,

suiiicient ammonia is added, in the preferred method of operating the present process, to bring the pH of the solution into the range of about 5.5 to 6.0. The use of an interneutralization tank for this purpose standardizes the conditions inthe reactor-carbonator to a desirable extent, simplifies the design of the reactor-carbonator, and facilitates control over operating variables. Some ferrous hydrate is precipitated in the interneutralization tank when the pH of the solution rises higher than 6.0. Therefore, it is highly desirable that the pH of the solution inthe interneutralization tank be maintained at as low a level as is consistent with a good balance of ammonia addition between the interneutralization tank and the injection of the reactor-carbonator. 5.9 has been found to fbe the optimum. The effluent from the interneutralizationv tank which has been referred to, for convenience, as a feed stock liquor, is united with a fairly large amount of recycled eluent from the alkaline section of the reactor-carbonator, and is injected immediately into the alkalinesection. Although the pH of the combined streams of feed stock liquor and recycled effluent from the alkaline section is such that this solution has a pH in the neighborhood of 7 to 8, the time of contact between the two solutions before injection in the reactor-carbonator is extremely brief, and this alkaline feed liquor immediately makes contact with carbon dioxide, so that a carbonate precipi- A pH of about` r1t-7 it@ farms. During the itypical operations, the alkaline feed liquor will contain np to 2.0 .grams per liter of free ammonia equivalent,` and vanywhere between `15 and .20! grams per liter 'of ferrousisulfate equivalent. The alkaline freed liquoralso contains substantial amounts O'Lerous Carbonate, ferioussulfate, ammonium sulfate, ferrous. ammonium sulfate, and ammonium hydroxide'. The recycle ratio ,of etuent f roxnthe alkaline section may vary from about `1"0`%`b`y .volume of` theeuent up Ito about 95% byvolume. A range of l50% to,f90.% is generally preferable.

In order ioincrease the throughput of alkaline freed liquor-"inA thelalkaline of the reactor-carbonated it is possible to increase die recycle ratio loto ,thev upper ofabout 95% by volume, as just mentioned. It .is also possible-to recycle into Ythe`interneu'tralization tank some of the decanted, cianiedfliquor'obtained ,from the decanter 57. The decanted' liquor has been clarified by `the rfioval of the Vferrous carbonate precipitate, and the recycling of a portion` of this liquor increases the volumenof liquido passing through the alkaline section, thus idecreasing the concentration of iron carbonateple cipitatewittiout diluting the ammonium sulfate solution. Where ,the cosiiceiltrationJ ofironrcarbonate the alkaline fedlicisor is Such that Spiayiasbsemas iiiiicult recycling `apportion of the `c iecanted liquor has many ad- Lvaiitases. @he such advantage isili@ thinning 0f the etiinent slurry.` `Pumping .is considerably facilitated where this slurry is thinned byihepresence'of a recycled porh tion ofthe decantad liquon Ihre p,reduction of a ,fast settling, easily filtered iron carbonate iprecipitate.in `the alkaline section A:of the reactoncarbonator requires Lcareful control over reaction conditions, such asthe `contact time 4in the `reactor-en bonator, the solution and gas temperatures, Ithe recycle ratios, and the gas composition. The pressure in the reactor-carbonator is also an important control variable. .While the preceding examples and general description have related to operations conducted at atmospheric pressure, a more effective formation of ammonium carbonate in the alkaline section is obtained when the preissure in the tower is superatmospheric. As will be pointed out, `the formation of ammonium carbonate is `believed .t0 b c the rst of a series-ot reactions which take place in the .reactor-carbonator. A pressure of between 3 and 5 atmospheres is sufficient to enhance this Areaction rate `without .unduly increasing the cost ofthe equipment. n

i Where a proper balance of ammonia addition .between the imei-neutralization `tank 4,and ,the reactor-carbonato;- is obtained, it is ,fo-oundthat the volume percent of carbon dioxide should closelyparallel to the volume percent of ammonia in the input gas. Thus, Where the input gas contains about'1`O%"by volume carbon dioxide, an amount of `ammonia. equivalent to about 9% by volume of ammonia generally suflices lfor a typical spent pickle liquor feed derived from a-sulfuric acid pickling solution. Simi- 1arly, where the input gases `consist essentially of about 21% byvolume of carbon` dioxide, the amount of ammonia which should bie-employed is in 4the neighborhood of an ecpiivalent 'of about 20% by volume of ammonia. The ammonia and carbon dioxide go into solution extremely rapidly, and react k to fform ammonium carbon;-l

ate. At or about 58 C., ammonium carbonate decoinposes into ammonia and carbon dioxide. Ammonium bicarbonate decomposes slowly at 38 C., and completely at 60? C.,` breaking up into thecarbonate, carbon dioxide and water. `Careful temperature control in ,the reactor-carbonator is therefore important. -Heat exchangers are desirably provided to maintain the liquid temperatures `below 6,0? C. i Higher temperatures cause ,undesirable decomposition '.lfhe carbonate ionY in. sointion reactswith the .ferrous ion ,inisolution to form inf soluble ferrous carbonate, which precipitates immediately drives the reaction t o completion. The presence of s tightened fatnaosiiimiivdroxids favorieten mation of Iferrous carbonate and tends to minimize the vformation` of ferrous bicarbonate. However, an unduly concentrationof ammonium hydroxide promotes the -formation of iron hydrate, which is undesirable. Close control over the amount of 4free ammonia everywherein, the system is therefore important. A consideration 'of these land other factors indicate that low operating temperatures, increased masses of reactants, and increased pressure, all favor the desired reaction in the alkaline section o'f .thefreactor-carbonator.

The'tempeiature may be controlled principally by the use of heat exchangers for the alkaline feed liquor. The temperature of the input gas may also be controlled. The concentration of ammoniaat any point in the reactorcarbonator may be controlled, to some extent, by a proper location of injectors for the ammonia within thereactorlcarbonator. For example, an ammonia injector may be positioned directly under each alkaline feed liquor spray inlet in the reactor-carbonator. In 'the simplest form of operation, equivalent amounts of ammoniafare injected in each of these locations. Howover, highly desirable results are obtained Where larger amounts are injected in the lowermost locations. This vir1`cre'g`1ses"the time of contact between the ammonia, the carbon dioxide, and `thealkaline feed liquor. Control over the pH of `the alkaline feed liquor is `also a signiiicant factor in obtaining an iron carbonate precipitate of desirable physical characteristics. The s'ir'nplest effective control may be obtained through the `iecyciiilg of liquid eiliuent from the reactor-carbonator, as hasbeen pointed out previously. This provides a practical control over the pH, vwithout the `undesirable formation of hydrate slirnes which would be obtained if that` same pH were obtained through the addition of ammonia .the interneutralization tank.

Where the system is operated, as is desirable, so that the gases discharged from the reactor-carbonator contain Lbetween 3 and 5 volume per cent of carbon dioxide, some advantages may lbe obtained by recycling a portion of these discharge gases through the reactor-carbonator. Such a recycling dilutes the gas feed to the reactor- `carbonator and `tends Ato prevent the formation of ammoninm `bicarbonate in the alkaline section. The increased `volume also provides for a greater degree or turbulence in the reactor-carbonator, and thus provides a more intimate contact between the liquids and gases in countercurrent flow. Y In thedecanter, the iron carbonate settles out as a gelatinous, amorphous,- semi-colloidal mass having a large surface. 4This mass has `the unique characteristic ofabsorbing and scavenging inhibitors from the liquor. Ihese inhibitors are those which are customarily added to the picklingsolutions, and which are carried overinto `the spent pickle liquor. Unless removed in some way, the inhibitors ordinarily contaminate ammonium sulfate recovered from spent lickle liquor. Ordinarily, these inhibitors are heterocyclic nitrogen compounds, and their presence in ammonium sulfate is undesirable. .Traceelements are also scavenged by the semi-colloidal ferrous carbonate precipitate in the decanter. For example, manganese and copper carbonates are relatively insoluble, inthe presence of a small excess of Vfree ammonia, and are removed in the decanter by the scavenging action of the ferrous carbonate.

The iron carbonate lter cake obtained by filtration of the vslurry from the decanter is a very valuable product. Thelter cake may be calcined to produce pigments or abrasives. It may also'be used as Va wet cake directlyin the wet'purication of gases. The Wet filter cake is extremely eifective in removing hydrogen sulfide from igases. 'ilhe filter cake may also be oxidized in air to .material which is efficacious for the removal for 'the removal of hydrogen suliide in the dry purification oi fgas. Alternatively, the air-oxiined filter cake may be mixed with shavings, orother suitablel diluents or binders, to produce products usable in the luuidized purification of gas. The filter cake may also Tbe sintered, with the simultaneous recovery of carbon dioxide and an iron product useful in the production of metallic iron. Any carbon dioxide recovered from the filter cake may be used directly in the reactor-carbonator. The value of the iron carbonate `filter cake Yis severely diminished where the lter cake nis contaminated by the presence of ironsuliide, as will be readily appreciated.

The decanter is merely a means for separating a thick slurry of ferrous carbonate from soluble-iron contaminated ammonium sulfate-solution. Consequently, Van

equivalent piece of apparatus, such as a centrifuge, Vthickener, or clarifier could be used in place of the decanting tank. However, settling in a tank olers simplicity of operation and ease of maintenance. Further concentration or thickening of the slurry from the decantation tank may be obtained through the use of a hydro-cyclone, which iwould provide a thickened slurry for the filter.

In order to provide uniformity of settling rates throughout the `change of seasons, it is desirable that the feed line to the decanter be provided with a heat exchanger. Through proper use of the fheat exchanger, it is possible to maintain 'a uniform temperature all year around, thus standardizing operating conditions. The pH of the liq- -uor in the `decanter should be maintained inthe range of about 9 lto 9.5, for Vbest separation of the iron carbonate precipitate. This, of course, is controlled through the regulation of lchearnount of ammonia which is employed to treat the spent pickle liquor inthe reactor-carbonator. F[The clarified liquor from the decanter containsI a small amount` of soluble iron. This soluble iron is removed in the sulliding tanks. The s-mall amount of hydrogen sulfide which is yemployed inthe suliding operation is insuicient to change the pH of the solution to any significant degree from its input pH of 7 to 9.5.

Although many types of apparatus could be employed for carrying out-the suliidingV operation, it is preferred to use settling tanks. The iron sulde remaining at thebottom of the settling tank after decantation of the supernatant-liquor frequently-has a tendency to adhere to the Walls of the tank. This undesirable tendency may be avoided by agitation of the sulde through the injection of inert gas to a'sparger at the bottom of the tank. The agitated material forms a slurry which is easily removed. i

The clarified liquor produced by the process is a valuable source of iron-free ammonium sulfate. One final chemical reaction takes place to insure maximum recovery of ammonia: the pH is adjusted by iron-free sulfuric acid. This produces a solution ideally suited for recovery of salable ammonium sulfate solids.`

It will be understood that the above description and examples have been given by way of illustration only, and are not intended to limit the scope of the invention in any way.

I claim:

1. A process for producing iron carbonate and ammonium sulfate from spent pickle liquor which comprises introducing into a reactor-carbonator zone an alkaline feed liquor (A) having a pH of about 7.0 to 8.0 and containingV ferrous ions, sulfate ions and free ammonia, contacting said feed .liquor (A) in saidrzone with gas- Veous ammonia and a gas consisting essentially of from about to 50 percent by volume carbon dioxide, said gas being cyanide-free and sulfide-free and containing less than about 4 percent byl volume oxygen land less than 2 percent by volume sulfur dioxide to produce a liquor (B) containing dissolved ammonium sulfate and an insoluble iron carbonate precipitate, the volumeipercent of`gaseo'us ammonia being about equal the volume percent of said carbon dioxide, adding ammonia to acidic liquor, (E) containing ammonia and spent pickle liquor vto produce a feed stock liquor (S) having a pH 20 in the range of 5.5 to 6.0,mixing` a portion of said liquor (B) with said'feed stock liquor (S) to produce said `feed liquor (A) immediately prior to introducing said feed "liquor (A) .into said reactor-carbonator zone, separating and recovering'the insoluble Yiron carbonate precipitate from the balance of, said `liquor `(B), and thereafter recovering iron-free ammonium sulfate from the kliquor remaining after said Separation. Y 2. ,The process of claim lwherein saidgas consisting essentially of carbon dioxide contains from about 8 to 25 percent by volume carbon dioxide and lthe volume percent of gaseous ammonia is about equal the volume percent of carbon dioxide.

3. A process for producing iron carbonate and ammonium sulfate Vfrom spent pickle liquor which comprises introducing into a reactor-carbonator zone an alkaline feed liquor (A) having a pH of Iabout 7.0 to 8.0, and containing lferrous ions, sulfate ions and free ammonia, contacting said feed liquor (A) lin said zone with gaseous ammonia and a gas consisting essentially of from about 8 to 25 percent by volume carbon dioxide, said gas being cyanide-free and sulfide-free and containing less than about 4v percent by volume sulfur dioxide to produce a liquor (B) containing dissolved ammonium sulfate and Van insoluble iron carbonate precipitate, the volume percent of gaseous yammonia being about equal thevolume percent vof said `carbon dioxide, mixing a portion of said liquor (B) with a feed stock liquor (S) 'having a ApH in the range of 5.5 to 6.0 to produce said feed liquor (A) immediately prior to introducing said feed liquor (A) into said reactor-carbonator zone, separating and recovering the insolubleiron carbonate precipitate from the balance of said liquor (B) to provide a separated liquor (D) containing dissolved ammonium sulfate, mixing a Vportion 'of said -liquor (D) with an acidic liquor (E) containing ammonia and spent pickle liquor and adding ammonia to said mixture of liquors (D) and (E) to produce said feed stock'liquor (S) having a pH in the range of 5.5 to 6.0, and recovering ironfree ammonium sulfate from the remainder of said separated Vliquor (D). v i `4. The process of claim 3 wherein a portion of said separated liquor (D) 'and ammoniaare mixed with said acidic -liquor (E) containing `ammonia and spent pickle liquor toV produce said feed stock` liquor (S) having a pH of about 5.9. l

5. The Vprocess of claim 3`V wherein saidseparated insoluble iron carbonate precipitate is washed with water anda Vportion of said separated liquor (D), ammonia and s-aid wash water are mixed with said acidic liquor (E) containing ammonia and spent pickle liquor to produce said feed stock liquor (S) having a pH of about 5.9.

6. `A process yfor producing iron carbonate and ammonium sulfate from spent pickle liquor which comprises introducing Vinto a reactor-carbonator zone an alkaline feed liquor (A) having a pH'of about 7.0 to 8.0 and containingV ferrous ions, sulfate ions and-free ammonia, contactingsaid feed liquor (A) in said zone with gaseous ammonia and a gas consisting essentially of from about 5 to 50 percent by volume carbon dioxide, said gas being cyanide-free vand -sulde-frecand containing less than about 4 percentby volume oxygenand less than 2 percent by volume sulfur dioxide vtolproduce a liquor (B) containing dissolved ammonium sulfate and an. insoluble `iron carbonate Y precipitate, -thevolumef percent of gaseousy ammoniabeing about 'equal-the volume percent of said carbon dioxide, discharging unreacted gaseous ammonia Vfromsaid reactor-carbonatorv zone into an 'acidic4 `absorption zone, introducingan acidic feed liquor (C) containing spent pickle liquor into `said ac id absorption` zone to absorb said discharged unreaeted gaseous ammonia, withdrawing an facidie liquor (E) containing spent pickle liquor'and absorbed ammonia from said acidic labsorption zone, adding addtionalammonia to said acidiefliquorKE) to produce la-feed' stock Y liquor (S) having a pH in the range of 5.5 to 6.0, mixing a portion of said liquor (B) with said feed stock liquor (S) to produce said feed liquor (A) having a pH of about 7.0 to 8.0 immediately prior to introducing said feed liquor (A) into said reactor-carbonator zone, separating and recovering the insoluble iron carbonate precipitate from the balance of said liquor (B), and thereafter recovering iron-free ammonium sulfate from the liquor remaining after said separation.

7. The process of claim 6 wherein a portion of the acidic liquor (E) containing spent pickle liquor and absorbed ammonia withdrawn from said acidic absorption zone is mixed with spent pickle liquor to produce said acidic feed liquor (C).

8. The process of claim 6 wherein said acidic liquor (E), said feed stock liquor (S) and said liquor (B) during the separation of the insoluble. iron carbonate precipitate therefrom are maintained under a blanket of 22 gaseous carbon dioxide to minimize the exposure of said liquors to oxygen.

9. The process of claim 6 wherein the liquor remaining after the separation of the insoluble iron carbonate precipitate from said balance of said liquor (B) but before the recovery of the ammonium sulfate is maintained under a blanket of gaseous carbon dioxide to minimize the exposure of said liquor to oxygen and gaseous hydrogen sulfide is introduced into said liquor to precipitate any traces of iron therein as iron sulide, separating said iron sulfide from said liquor.

References Cited in the le of this patent UNITED STATES PATENTS 2,511,306 Tiddy June 13, 1950 2,511,307 Tiddy June 13. 1950 2,775,508 lThomsen Dec. 25. 1956 

1. A PROCESS FOR PRODUCING IRON CARBONATE AND AMMONIUM SULFATE FROM SPENT PICKLE LIQUOR WHICH COMPRISES INTRODUCING INTO A REACTOR-CARBONATOR ZONE AN ALKALINE FEED LIQUOR (A) HAVING A PH OF ABOUT 7.0 TO 8.0 AND CONTAINING FERROUS IONS, SULFATE IONS AND FREE AMMONIA, CONTACTING SAID FEED LIQUOR (A) IN SAID ZONE WITH GASEOUS AMMONIA AND A GAS CONSISTING ESSENTIALLY OF FROM ABOUT 5 TO 50 PERCENT BY VOLUME CARBON DIOXIDE, SAID GAS BEING CYANIDE-FREE AND SULFIDE-FREE AND CONTAINING LESS THAN ABOUT 4 PERCENT BY VOLUME OXYGEN AND LESS THAN 2 PERCENT BY VOLUME SULFUR DIOXIDE TO PRODUCE A LIQUOR (B) CONTAINING DISSOLVED AMMONIUM SULFATE AND AN INSOLUBLE IRON CARBONATE PRECIPITATE, THE VOLUME PERCENT OF GASEOUS AMMONIA BEING ABOUT EQUAL THE VOLUME PERCENT OF SAID CARBON DIOXIDE, ADDING AMMONIA TO AN ACIDIC LIQUOR (E) CONTAINING AMMONIA AND SPENT PICKLE LIQUOR TO PRODUCE A FEED STOCK LIQUOR (S) HAVING A PH IN THE RANGE OF 5.5 TO 6.0, MIXING A PORTION OF SAID LIQUOR (B) WITH SAID FEED STOCK LIQUOR (S) TO PRODUCE SAID FEED LIQUOR (A) IMMEDIATELY PRIOR TO INTRODUCING SAID FEED LIQUOR (A) INTO SAID REACTOR-CARBONATOR ZONE, SEPARATING AND RECOVERING THE INSOLUBLE IRON CARBONATE PRECIPITATE FROM THE BALANCE OF SAID LIQUOR (B), AND THEREAFTER RECOVERING IRON-FREE AMMONINIUM SULFATE FROM THE LIQUOR REMAINING AFTER SAID SEPARATION. 